Data Structure C program Stack using Array Dev c++

 #include <stdio.h>  
 #include <conio.h>  
 #define MAX 20  
 int top=-1,ch;  
 int a[MAX];  
 int main(){  
   void menu();  
   void push();  
   void pop();  
   void display();  
   menu();  
   while(ch!=3){  
      switch(ch){  
           case 1:  
               push();  
               display();  
               break;  
           case 2:  
               pop();  
               display();  
               break;  
           }  
      menu();       
      }  
 }  
 void push()  
 {  
    if(top==MAX-1)  
     printf("Stack is full\nStack overflow\n");  
    else  
    {  
     printf("Enter element:");  
     scanf("%d",&a[++top]);  
    }  
 }  
 void pop()  
 {  
    if(top==-1)  
    printf("Stack is emplty\nSTACK UNDERFLOW\n");  
    else  
    {  
      top--;  
      printf("Deleted %d\n",a[top+1]);  
    }  
 }  
 void display()  
 {  
    int i=0;  
    printf("\nStack is :\n");  
    for(i=top;i>=0;i--)  
    printf("%d\n",a[i]);  
 }  
 void menu()  
 {  
    printf("\n1.Push\n2.Pop\n3.Exit\n");  
    scanf("%d",&ch);  
    while(ch<1 || ch>3)  
    {  
     printf("Enter valid choice 1,2 or 3\n");  
     scanf("%d",&ch);  
    }  
 }